According to ratings and reviews in the industry WHMCS is the leading software however there are many other software that you can count own
Listing some of the popular software:
- WHMCS
- ClientExec
- WHM AutoPilot
- AWBS
- Blesta
- HostBill
- Plesk Billing (ModernBill)
Group A: Include: WHMCS, WHMAP, ClientEXec, Blesta
Group B: AWBS, HostBill
Group C: Plesk billing
Group A winner is definitely WHMCS because it's more advanced in modules, hooks, addons and other support
Group B AWBS is a good solution for Domain Registration and some other services because of more features included, HostBill compared to WHMCS is somehow the same but HostBill has more advanced modules and sometimes they are the first for new ideas for example they were the first billing providing VPS order slider and later WHMCS made that.
Group C Plesk is alone and the reason for that simply if you are providing Parallels products and services so definitely you should choose Plesk for example if your are providing Plesk Windows or Linux hosting or Helm hosting don't think twice Plesk billing is the best but it's not easy to use and configure.
At the end WHMCS has the largest community but not the first in the industry because WHMAP was the first but WHMAP lost so much because of their delay in development. WHMCS is used by most providers, WHMCS is more expensive from others, WHMCS has more strict rules than others and WHMCS has more 3rd party modules than others.
To know the best for you first you should know what type of services you like to provide if it's the traditional CentOS with cPanel/WHM go with WHMCS. If you want more advanced in options specially regarding Domain Management AWBS, if you need something like WHMCS easy-to-use and sometimes get new ideas before others do go with HostBill. For Parallels products your choice is Plesk Billing.
For us we use WHMCS, Plesk Billing, HostBill, ISPBilling & at some places with use AWBS.